The delivery of radio frequency (RF) energy to target regions within tissue is known for a variety of purposes. Of particular interest to the present invention, RF energy may be delivered to diseased regions in target tissue for the purpose of causing tissue necrosis. For example, the liver is a common depository for metastases of many primary cancers, such as cancers of the stomach, bowel, pancreas, kidney, and lung. Electrosurgical probes with deploying electrode arrays have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. See, for example, the LeVeen™ Needle Electrode available from Boston Scientific Corporation, which is constructed generally in accord with U.S. Pat. No. 6,379,353, entitled “Apparatus and Method for Treating Tissue with Multiple Electrodes.”
The probes described in U.S. Pat. No. 6,379,353 comprise a number of independent wire electrodes that are deployed into tissue from the distal end of a cannula. The wire electrodes may then be energized in a monopolar or bipolar fashion to heat and necrose tissue within a defined volumetric region of target tissue. Difficulties have arisen in using the multiple electrode arrangements of U.S. Pat. No. 6,379,353 in treating tumors that lay at or near the surface of an organ, such as the liver. Specifically, some of the tips of the electrode array can emerge from the surface after deployment. Such exposure of the needle tips outside the tissue to be treated is disadvantageous in a number of respects. First, the presence of active electrodes outside of the confinement of the organ being treated subjects other tissue structures of the patient as well as the treating personnel to risk of accidental contact with the electrodes. Moreover, the presence of all or portions of particular electrodes outside of the tissue being treated can interfere with proper heating of the tissue and control of the power supply driving the electrodes. While it would be possible to further penetrate the needle electrode into the treated tissue, such placement can damage excessive amounts of healthy tissue.
In response to these adverse results, a device for ablating a tumor at or near the surface of tissue has been developed. Specifically, as illustrated in FIG. 1, an ablation assembly 20 comprises a surface electrode 22 and an electrosurgical probe 24, such as a LeVeen™ electrode, that can be operated in a bipolar mode to ablate the tissue in contact with, and between a needle electrode array 26 mounted to the distal end of the probe 24 and the surface electrode 22. As illustrated, the surface electrode 22 comprises a generally flat or planar disk-shaped plate 28 and a plurality of tissue penetrating electrodes 30 that project perpendicularly from the plate 28. The surface electrode 22 further comprises a central aperture 32 that extends through the plate 28, so that the surface electrode 22 can be threaded over the probe 24 and locked into place about the deployed probe 24. The ablation assembly 20 can then be operated in a monopolar or bipolar mode to ablate the tissue in contact with, and between, the electrode array 26 of the probe 24 and the needles electrodes 30 of the surface electrode 22. Further details regarding these types of ablation devices are disclosed in U.S. Pat. No. 6,470,218, entitled “Apparatus and Method for Treating Tumors Near the Surface of an Organ,” which is hereby fully and expressly incorporated herein by reference.
Although the ablation assembly illustrated in FIG. 1 is generally useful in ablating superficially oriented tumors, it cannot be used to efficiently and safely ablate such tumors in all circumstances. For example, if the tumor is quite close to the surface of the tissue, placement of the needle electrodes without exposing any metallic surface can be difficult. Also, it may not be practical to use the probe assembly when the tumors are quite shallow. In this case, the surface electrode may be used by itself. Efficient ablation of the tumor, however, may not be achieved if the tumor has a non-uniform thickness.
There thus is a need to provide improved systems and methods for more efficiently and safely ablating superficially oriented tumors.